The application generally relates to compressors for vapor compression systems. The application relates more specifically to a system to maintain the position of a valve in a positive-displacement compressor.
A vapor compression system includes a compressor that draws gas into a suction inlet, compresses the gas to increase the pressure of the gas, and then discharges the compressed gas at a discharge outlet. The compressed gas from the compressor then flows to another component of the system. The component to receive the compressed gas can be a pipeline, a storage container, a heat exchanger, or any other suitable component depending on the application of the vapor compression system. The gas used in the system can be a natural gas, for example, methane, ethane, propane, and butane; an industrial processing gas, for example, carbon dioxide, oxygen, nitrogen, helium, and argon; a refrigerant, for example, ammonia, carbon dioxide, or hydrofluorocarbon-based refrigerants (for example, R410A); and/or air.
In positive-displacement compressors, capacity control may be obtained by both speed modulation and suction throttling to reduce the volume of vapor or gas drawn into a compressor. Capacity control for a compressor can provide continuous modulation from 100% capacity to less than 10% capacity, good part-load efficiency, unloaded starting, and unchanged reliability. In some positive-displacement compressors, capacity can also be controlled by a slide valve employed in the compressor. The slide valve can be operated to remove a portion of the vapor from the compression chamber of the compressor, thereby controlling the capacity of the compressor. Besides the slide valve, other mechanical devices, such as slot valves and lift valves, may be employed in positive-displacement compressors to control capacity. Adjustments to capacity control valves or variable displacement mechanisms can meet the demands of the system. In a refrigeration system, capacity can be regulated based upon a temperature setpoint for the space being cooled. In other systems where the compressor is processing gas, capacity may be regulated to fully load the torque generator or prime mover (turbine or engine drive) for the compressor.
In natural gas applications, vapor compression systems can be used at the point the natural gas is recovered, for example, at the well head, and to maintain an appropriate level of pressure to maintain flow along the pipelines, for example, at a distance of about every 40 to 100 miles along a pipeline.
In some natural gas applications, the vapor compression system may be in a remote area. One problem with locating a vapor compression system in a remote area is that electrical power may be unavailable or difficult to generate. Furthermore, electrical power may not even be desired for natural gas applications (whether or not electrical power would be available) due to a risk of fire and/or explosion from the combustible fluid being worked and the possibility of sparks from the electrical connections, for example, solenoid valve connections. Thus, the efficiency of a remotely located vapor compression system may be reduced due to an inability to control the capacity of the compressor from a lack of electrical power.